Deploying a service is necessary to make it available to users. During development, a service is created and tested in a test environment. After successful testing, the service is deployed to the production system where it can be run by the appropriate user groups.
Especially in fusion teams, developers want to make a finished feature or an intermediate state of a service available to other team members for testing while they continue to work on the same service. This is easily possible in the Designer by using the test environment for developing, and deploying to the test server only when necessary. You only need to deploy your service if you want to allow other team members to test new features or if you want to run regeression tests against your test server.
For further details on testing and deploying, refer to Working with the Test Environment and Deploying a Service in the Designer guide.